1. Field of the Invention
This invention relates generally to membrane elements for use in reverse osmosis or ultrafiltration separation devices and more particularly, to a spiral wound membrane element product water tube with external channels directing flow of permeate to apertures in the tube.
2. Description of Related Art
The use of semipermeable membranes for effecting liquid separations has become well accepted, and membranes both for ultrafiltration and reverse-osmosis are presently in use for a wide spectrum of applications, including water purification, concentration of dilute mixtures or solutions, waste treatment and/or recovery of recyclable components. A large variety of semipermeable membranes has been developed, and spiral-wound elements have become one accepted type of device which has been found valuable for efficiently providing a large amount of membrane surface area within a given spatial volume.
Referring to FIG. 9, a typical prior art spiral wound reverse osmosis element 10 is shown. The element 10 is formed by wrapping one or more membrane leaves 12 and feed spacer sheets 14 about a central product water collection tube 16. The leaves 12 comprise two generally rectangular membrane sheets 18 surrounding a permeate carrier sheet 20. This “sandwich” structure is commonly held together by adhesive along three edges, although several other means exist to seal the three leaf edges.
The fourth edge of the leaf 12 abuts the product water collection tube 16 so that the permeate carrier sheet 20 is in fluid contact with rows of small holes 22 in the product water collection tube 16. An initial wrap of the permeate carrier sheet 20, typically consisting of between two and four layers of the permeate carrier, is wrapped around the product water tube 16 before insertion of the first membrane leaf 12. This initial wrap of permeate carrier 20 supports the membrane leaf 12 over the holes 22 and provides a path to conduct permeate water from the membrane leaves 12 to the holes in the product water collection tube 16.
The holes 22 typically have a diameter of about 0.125 inches (3.18 mm) to conduct permeate from the permeate carrier 20 wrapped around the outside of the product water collection tube 16 to the center of the tube 16. The holes 22 in the product water collection tube 16 are designed to channel permeate from the outside to the inside of the tube 16 with minimal pressure drop. The maximum size of the hole 22 is limited due to the tendency of the membrane leaf 12 to be drawn into the holes 22 and being damaged. The minimum size and number of holes 22 is limited by the pressure drop created by permeate water flow through the holes 22. The maximum number of holes 22 is limited by manufacturing costs for the product water collection tube 16.
Each leaf 12 is separated by feed spacer sheet 14 that is also wound about the product water collection tube 16. The feed spacer 14 is in fluid contact with both ends of the element and it acts as a conduit for feed solution across the surface of the membrane sheets 18. The direction of feed flow is from the entrance end 24 to the concentrate end 26 and this direction is parallel to the axis A of the central product water collection tube 16.
It would therefore be desirable to have an element design that would aid the flow of permeate from the membrane leaves into the product water collection tube. It also would be desirable to reduce the number wraps of permeate carrier around the product water tube.